The first thing to remember when dealing with shock hazards is that you need to contact two points that have a voltage difference between them to be in danger. Voltage difference is what causes current flow, and current flow is what is dangerous.
If you touch the hot lead on an electrical cord while you are insulated from ground or any other conductive object, you will not be shocked. You can touch two hot leads from the same source, one in your hand and the other with your tongue (don't try this at home) and you won't be shocked because they are both at the same voltage.
As a greatly simplified thought model, imagine that all possible electrical sources that could contact your pool are represented by a simple extension cord with bare wires on the end. The hot hanging into the pool, and the ground connected to a ground rod. The cord isn't plugged in, so it isn't doing anything, but an accident could happen and it could get plugged in causing current to flow through the pool and/or equipment.
If the hot is in the water and the cord is plugged in, the water will rise to 120VAC, and any swimmers will too. But voltages are all relative. We may be at 5,000,000V compared to the surface of Saturn, but as long as we don't touch the surface of Saturn, nothing will happen. That 120VAC is only important relative to the neutral and ground of the same electrical system. Ground is just as dangerous to touch when you are at 120V as 120V is when you are at ground potential.
What about the pool walls of a metal pool? They are sitting on the ground, so they are at zero volts relative to the pool water because the liner insulates them from the water. If a swimmer touches the metal wall, current will flow between their hand and the pool water. Same thing if somebody is getting ready to clean the pool. They walk up and lean against the pool wall at zero volts, and then touch the pool water with a hand or a metal pole. Current flows.
The source of power may have a GFCI, but how do you know? It could be your crazy neighbor who ran an extension cord into his trees for mood lighting, or a previous owner of the home who didn't wire your outlets right. How can you protect against such things that you can't control or predict? The easy way is to connect everything conductive that you can touch from the pool together with a wire. If every conductive object a swimmer can touch is at the same voltage or very close, very little current will flow and the swimmers or pool cleaner will be safe.
Bonding is the process of connecting everything conductive that a swimmer might touch from the pool together with a wire so that no two places the swimmer can touch are at different voltages.
